To study the regulation of invertebrate muscle, we isolated myosin both from the striated muscle of Limulus - the horseshoe crab and from the flight muscle of Lethocerus - a water bug, and examined the sliding velocity of actin filaments over the purified myosins. Little is known about the regulation of Lethocerus flight muscle. Our preliminary results show that the thick (phosphorylation) filament regulatory system may play an important role in Lethocerus flight muscle. Limulus striated muscle is known to be regulated by both the thin (troponin and tropomyosin) and thick (phosphorylation) filament regulatory systems which are calcium- dependent. Our experiments demonstrated that the "off" state of either system is dominant and for the movement of actin filaments to occur, both phosphorylated Limulus myosin and an activated troponin and tropomyosin system are required. Tropomyosin from different sources appeared to increase the sliding velocity 5-10 fold when bound to actin. Calcium does not alter the velocity significantly in the absence of both troponin and tropomyosin. Partial sequences of the tryptic phosphopeptides of Limulus myosin light chains following the phosphorylation by gizzard myosin light chain kinase yield ATS(PO4)NVFAMFEQNQIA for the 21 kD, and SGS(PO4)NVFSMFTE for the 31 kD light chain. Comparison of sequences suggests that the sequence around the phosphorylation site of Limulus light chains is more similar to that of vertebrate smooth muscle myosin light chain than to that of vertebrate striated muscle myosin light chain.